Armstrong, who was last of the 148-strong field to ride down the ramp and into torrential rain, started the day with a 1min 05sec lead over German rival Jan Ullrich who was hoping to reduce the deficit ahead of Sunday's 20th and final stage.
However Ullrich, despite a fast start, failed to repeat his earlier time trial victory over the 31-year-old American champion who held on to finish in third place behind Millar and compatriot Tyler Hamilton, who was second.
Armstrong came over the line 14secs behind Millar's winning time of 54min 30secs, punching the air in victory after having avoided the fate of Ullrich who had crashed in the tricky latter stages of the course.
"I was relieved to make it through a time trial that was very dangerous at the end and relieved to have got so much closer to winning the Tour de France, " said Armstrong.
"I was extremely nervous this morning. I even tried to take a little nap this afternoon but I couldn't.
"There was no point in me taking risks with the wind and the rain like that. Then when I heard that Jan had fallen I just told myself, 'keep calm'.
"It wasn't important for me to win today. It was just important to stay on the bike."
Given the stakes -- joining the exclusive club of five-time Tour de France winners -- his anxiety is understandable.
Armstrong will join Frenchmen Jacques Anquetil and Bernard Hinault, Belgian Eddy Merckx and Spaniard Miguel Indurain in the five-time winners' club.
And if he stays out of trouble on Sunday's stage Armstrong will also equal Indurain's record of winning the race five years in a row (1991-95).
The 31-year-old US Postal rider now has a virtually unassailable lead of 1: 16 over Ullrich, the 1997 champion who is poised to come runner-up for a fifth time.
Ullrich had a slim chance of overtaking Armstrong at the last hurdle if he managed to pull off another stunning time trial win.
However, the 29-year-old Bianchi rider, who beat Armstrong by a massive 1min 36sec in the 12th stage time trial in Toulouse, was never a danger to the American.
The German, who started the stage three minutes earlier, took a few seconds off Armstrong in the first part of the course but was less quicker on the second.
And when he came sliding off his bike after failing to negotiate a left hand bend on one of the many roundabouts, his chances of making up any time disappeared.
Ullrich admitted defeat, but his surprising second place finish in the race looks promising for the future.
"I'm disappointed with not winning the stage," said Ullrich, who was left with a heavily bleeding right forearm and scrapes on his leg after his crash.
Ullrich had come into the race after a few chaotic months of preparation and had simply been hoping for a stage win before making a bid to win the race for a second time next year.
"Overall I'm pretty satisfied. Even before the crash I knew I wasn't going to gain more time on Armstrong," he admitted.
The careful approach of both Ullrich and Armstrong left Millar with a safety margin after his blistering time had separated the contenders from the pretenders.
And it more than made up for the big Cofidis rider's disastrous Tour de France so far.
"I'm in heaven," said Millar, who almost quit the race a few days ago before his team-mates convinced him he could still pull off a late win to make up for his opening prologue disappointment where he lost to Aussie Brad McGee by eight hundredths of a second.
"I was totally relaxed and wasn't really caring too much about the result. I just went out and did my best."
Referring to his nightmarish time in the Pyrenees which almost ended his race prematurely, Millar said: "I found the joy of riding a bike again."
Armstrong meanwhile admitted it had been the hardest and most challenging Tour win he has ever faced in his 12 years of riding in Europe.
After facing the threat of on-form Spaniard Joseba Beloki, who crashed out of the race early on, then dealing with the unexpected arrival of Ullrich, he said that US Postal's strategy had played a major role in his success.
"This year we had to rely more on strategy than on physical gifts or fitness," said Armstrong.
"But it's good for me to have a tough year for a change because with victories of five and six minutes you can start to take things for granted.
"This year was different and guys like Beloki and Ullrich are for real. Beloki crashed out but could have caused problems and Jan is still a big champion.
"But I won't make the same mistakes again."
Stage 19 results, top 15 (Pornic -- Nantes, 49 km ITT):
1. David Millar (GBR/COF) 54min 05sec
(average: 54.361 kph)
2. Tyler Hamilton (USA/CSC) at 0:09
3. Lance Armstrong (USA/USP) 0:14
4. Jan Ullrich (GER/BIA) 0:25
5. Laszlo Bodrogi (HUN/QST) 0:26
6. Viacheslav Ekimov (RUS/USP) 0:56
7. Victor Hugo Pena (COL/USP) 1:00
8. George Hincapie (USA/USP) 1:08
9. Sylvain Chavanel (FRA/BLB) 1:12
10. Marzio Bruseghin (ITA/FAS) 1:26
11. Stuart O'Grady (AUS/C.A) 1:38
12. Michael Rogers (AUS/QST) 1:40
13. Christophe Moreau (FRA/C.A) 1:43
14. Laurent Lefevre (FRA/DEL) 1:46
15. Michael Blaudzun (DEN/CSC) 1:56
Stage 19 overall standings, top 15
1. Lance Armstrong (USA/USP) 80hr 02min 08sec
2. Jan Ullrich (GER/BIA) at 1:16
3. Alexander Vinokurov (KAZ/TEL) 4:29
4. Tyler Hamilton (USA/CSC) 6:32
5. Haimar Zubeldia (SPA/EUS) 7:06
6. Iban Mayo (SPA/EUS) 7:21
7. Ivan Basso (ITA/FAS) 10:12
8. Christophe Moreau (FRA/C.A) 12:43
9. Carlos Sastre (SPA/CSC) 18:49
10. Francisco Mancebo (SPA/BAN) 19:30
11. Denis Menchov (RUS/BAN) 19:44
12. Georg Totschnig (AUT/GRL) 21:47
13. Peter Luttenberger (AUT/CSC) 22:31
14. Manuel Beltran (SPA/USP) 23:03
15. Massimiliano Lelli (ITA/COF) 24:15